Ferrous metal article coated with an aluminum zinc alloy



Sept. 26, 1967 A. R. BORZILLO ETAL 3,343,930

FERROUS METAL ARTICLE COATED WITH AN ALUMINUM ZINC ALLOY Filed July 14,1964 3 Sheets-Sheet 1 INVENTORS Ange/0 R. Borz/l/o James B. Horton Sept.26, l A. R. BORZILLO ETAL 3,343,930

FERROUS METAL ARTICLE COATED WITH AN ALUMINUM ZINC ALLOY Filed July 14,1964 3 Sheets-Sheet 2 INVENTORS Ange/0 R. Borz/l/o James B. Horton p1967 A. R. BORZILLO ETAL 3,343,930

FERROUS METAL ARTICLE COATED WITH ANALUMINUM ZINC ALLOY I INVENTORSAnge/0 R. 50/2/7/0 James B. Hor/on United States Patent 3,343,930FERROUS METAL ARTICLE CGATED WITH AN ALUMINUM ZlNC ALLOY Angelo R.Borzillo, Allentown, and James B. Horton,

Bethlehem, Pa., assignors to Bethlehem Steel Company, a corporation ofPennsylvania Filed July 14, 1964, Ser. No. 382,595 9 Claims. (Cl.29-1962) This invention relates to zinc-aluminum coatings on ferroussurfaces, and more particularly to zinc-aluminum protective coatings onsteel surfaces which coatings are ductile, corrosion resistant andtightly adherent to said surfaces.

In the protection of ferrous surfaces from corrosive media, it is commonpractice to coat the ferrous metal surface with a non-ferrous metal, byimmersion in a molten bath of the coating metal or by other methods.These coatings should not only be protective, but they should be ductileand tightly adherent as well, and should have a smooth, pleasingappearance.

Many metals have been used in the past as a protective coating forferrous surfaces, such metals including tin, zinc, terne metal, nickel,chromium, cadmium, copper, aluminum, bronze and lead. One of the mostcommonly used coating metals is zinc, partly because of its relativelylow cost, and partly because of its higher position in the electromotiveseries relative to iron. However, for many applications, Zinc coatingsdo not have adequate resistance to corrosion. The search for metalliccoatings which will combine high protective power, ductility, adherence,good appearance and low cost is a continuing one.

Accordingly it is an object of this invention to provide a ferrous basehaving thereon a metallic coating which is highly corrosion-resistant.

Another object of this invention is to provide metallic coatings onferrous base stock, which coatings are metallurgically bonded to thebase stock.

Another object of this invention is to provide a ferrous base which hasa metallic coating thereon and in which the iron-bearing alloy layer atthe interface between coating and base metal is thin and uniform.

A further object is to provide a ferrous base having a metallic coatingthereon characterized by good appearance and formability.

An additional obiect is to provide a ferrous base with a metalliccoating which is ductile and firmly adherent.

Another object is to provide a method for producing said coated product.

We have discovered that the foregoing objects can be attained byproviding a ferrous base which has metallurgically bonded thereto acoating consisting essentially of from 25% to 70% aluminum, with thebalance substantially zinc.

Our product is characterized by the fact that the ironbearing alloylayer at the interface of the ferrous base and the coating is thin anduniform.

Briefly, the method by which our new product is obtained broadlycomprises applying to a ferrous base, such as strip, the surface ofwhich is substantially free from oxides and other contaminants, acoating consisting essentially of 25% to 70% aluminum, balance zinc, thecoating being applied in such a manner as to result in a thin anduniform iron-bearing alloy layer at the interface between the base andthe coating.

This invention has particular applicability for the coating of steelstrip and wire. Heavy alloying at the interface of steel strip and anon-ferrous protective coating metal promotes cracks in the coating uponsubsequent deformation of the coated product. In addition, with strip ofthin gages, heavy alloying of coating metal with iron of the 3,343,930Patented Sept. 26, 1967 strip results in a large proportionate loss ofbase metal, thus weakening the base metal itself. Then too, heavyalloying at the interface at times results in an uneven coating, thesurface appearance of which leaves much to be desired.

The coated product of our invention and a preferred method, by which thecoating may be applied to a ferrous base, will be described withreference to the following drawings.

FIG. 1 is a reproduction of a photomicrograph of a cross section of acoated steel strip embodying our invention, the coating containingapproximately 25% aluminum, and approximately 74% zinc.

FIG. 2 is a reproduction of a photomicrograph of a cross section of acoated steel strip embodying our invention, the coating containingapproximately 35% aluminum, and approximately 64% zinc.

FIG. 3 is a reproduction of a photomicrograph of a cross section of acoated steel strip embodying our invention, the coating containingapproximately 54% aluminum, and approximately 44.5% zinc.

FIG. 4 is a reproduction of a photomicrograph of a cross section of acoated steel strip embodying our invention, the coating containingapproximately 70% aluminum, and approximately 28% zinc.

FIG. 5 is a diagrammatic side elevation of a preferred embodiment ofapparatus for producing the product of this invention.

A preferred method of coating a ferrous base, e.g. steel strip, is asfollows. The strip is first passed through a cleaning solution whichremoves grease and dirt from the surface thereof. The cleaned strip isthen introduced into a furnace and heated to a temperature approximatelythat of the coating bath. The strip is then passed through a protectiveatmosphere directly into a bath of molten metal consisting essentiallyof 25 to 70% aluminum, balance zinc. The temperature of the bath ismaintained slightly above the melting point of the metal, the exacttemperature depending, of course, upon the relative amounts of aluminumand zinc in the bath.

Strip speeds and bath immersion times are similar to those used incontinuous galvanizing.

Following is a specific example of our preferred method.

Referring to FIG. 5, a strip 1 of 28 gage rimmed steel, having a carboncontent of 0.06%, a manganese content of 0.31%, and other elementscustomarily found in rimmed strip steel, was fed from pay-off reel 2 tocleaning tank 3 containing an aqueous solution 4 (approximately 4 oz./gal.) of Pennsalt No. 30, a standard alkaline cleaning solution forsteel strip. The cleaning solution was maintained at a temperature ofapproximately 200 F. The cleaned strip was scrubbed at scrubbers 5, andrinsed in tank 6 with water rinse 7. From the rinse tank 6, the stripwas led through rubber squeegee rolls 8, over rolls 9 and downwardlythrough a furnace 10, where the strip was heated to a temperature ofapproximately 1200 F. The furnace was heated by the products ofcombustion of natural gas and air in the ratio of 1 to 8. From thefurnace, the strip was led around positioning roll 11, through a holdingchamber 12 where its temperature fell to approximately 800 F. Anatmosphere of 99% hydrogen was maintained in the holding chamber 12 toprotect the strip from oxidation prior to its entry into the coating pot13. After entering pot 13, the strip was passed through a molten bath14, by way of sinker rolls 15 and 15. This bath was maintained at atemperature of 1060 F. Upon leaving the bath, the strip was passedbetween a pair of smooth low carbon steel exit rolls 16. An air-blast 17was used to chill the coating as the strip passed vertically to roll 18,and then take-up reel 19. The speed of the strip was approximately 45feet per minute and the time of immersion in the coating bath was about4 seconds. The total coating thickness, which includes the coating onthe two sides of the thus-coated strip, averaged 0.00197 inch.

In preparing the molten bath, 99.50% minimum Al grade aluminum was used.This material had the following analysis:

Percent Aluminum 99.65 Iron 0.19 Silicon 0.13 Manganese 0.01 Zinc 0.01Vanadium 0.01 Cadmium 0.02 Sodium 0.001

The zinc employed for the molten mixture had a specified zinc purity ofbetter than 99.99%.

Silicon was added to the bath in the form of an aluminum alloy. Thismaterial analyzed as follows:

Percent Silicon 12.00 Iron 0.3 3 Aluminum Balance These ingredients werecombined in such proportions that the bath analyzed approximately 35aluminum, 64% zinc, and 1% silicon.

A photomicrograph taken at 500 magnifications, of a cross section of thecoated product, made by the method of the example, is reproduced in FIG.2. The alloy layer between the coating and base metal is so thin as tobe almost indistinguishable at 500 magnifications.

FIG. 1 is a reproduction of a photomicrograph, also at 500magnifications, of a cross section of another coated product of ourinvention wherein carbon steel strip was coated by the process abovedescribed in a bath analyzing approximately 25 aluminum, 74% zinc and0.77% silicon. In this sample, there is no distinguishable alloyformation of iron with coating metal.

As the aluminum content in the coating is increased above 35%, there isevidence of a small amount of interfacial alloy layer. But even when thealuminum represents 70% of the alloy coating, the iron-bearing alloy atthe interface is uniform and thin. The thinness and uniformity of theinterfacial alloy occurring with the higher ranges of aluminum can beobserved in FIGURES 3 and 4, each of which is a photomicrograph, at 500magnifications, of a cross section of a similar steel, coated by theprocess above described. In FIG. 3, the bath, from which the coatedproduct was made, analyzed approximately 54% aluminum, 44.5% zinc and1.5% silicon, While in FIG. 4 the coating bath had an analysis ofapproximately 70% aluminum, 28% zinc and 2% silicon.

All of the samples shown in FIGURES l-4 were etched in a solution of 95%amyl alcohol, 5% nitric acid, for five seconds.

By thin iron-bearing alloy layer is meant a layer at the interfacehaving an average thickness not greater than about 0.25 mil, excludinglocalized projections. However, our invention, in its broad aspects, isnot limited to an alloy layer of this thinness, for in some applicationsit may be possible to tolerate a layer of greater thickness.

By uniform is meant a layer surface which is substantially flat (for aflat substrate) but which may include localized projections into thezinc-aluminum coating, as shown, for example, in FIGS. 3 and 4.

The operating temperature of the molten coating bath will ordinarilyrange from about 975 F. to 1220 F., depending on the amount of aluminumin the alloy, the temperature increasing with increasing aluminumcontent.

Specific examples of preferred bath temperatures for varying aluminumcontents are set forth in the following examples:

Approximate bath Aluminum (percent): temperature F.) 25 995 Coatingsmade as above described are metallurgically bonded to the ferrous base.

Our coated products have many favorable properties, as Will be apparentfrom the following data based on coatings produced by the method abovedescribed.

In the entire coating range of 25% to 70% aluminum, the coated productsof this invention show an advantage over continuously galvanizedproducts in the salt spray test of the order of approximately 3 to 1, upto 8 to 1. The salt spray test is is performed according to A.S.T.M.Method B117-62.

Typical results (average of four 4" x 6" test specimens with protectededges) for salt spray tests are set forth in the following table for agalvanized steel sheet, and for steel sheets coated with zinc-aluminumcoatings according to the invention.

TABLE I Hours to first (1%) rust per Type of coating: mil of coatingthickness (1) Continuous galvanized 325 (2) Zinc-aluminum coatingcontaining approximately 25 Al 2290 (3) Same as (2) with approximately35 Al 3234 (4) Same as (2) with approximately 45% A1 1 3805 (5) Same as(2) with 54% Al 1 3405 (6) Same as (2) with 70% Al 940 1 Tests notcompleted.

Appearancewise, the coated products of the invention exhibit a smallspangle. This property is highly desirable in many operations, whereadditional surface treatment, such as the application of paint, isrequired. In the coating range of from about 25% to 45% aluminumcontent, the surface of the product has an exceptionally smooth, whitelustre.

In the so-called muffier test, the corrosion resistance of the productsof our invention, within the coating range of 25 to 70% aluminumcontent, is far and above that which can be met with galvanizedproducts.

The muffier test referred to is that known as Mufiier Condensate Test.In the test, coated specimens, 2 inches by 4 inches, are dipped into anaqueous solution of 0.05 normal sulfuric acid and 0.01 normalhydrobromic acid for 8 seconds at a solution temperature of F. Thespecimens are removed at the end of the 8 second immersion period, andsuspended in the vapors of the immersion solution for the remainder ofone hour. The procedure is repeated every hour for 20 hours, after whichthe specimens are heated in a laboratory furnace for 2 hours at 500 F.to complete what is referred to as a one-day cycle. Specimens areexamined visually after each one-day cycle for start of rusting(disregarding the rusting of the sheared edges of the specimens). Thecomplete test comprises 13 one-day cycles.

Test results from the mufiler test are shown in Table II, which follows:

(3) Same as (2) with approximately 35% TABLE IIContinued Cycles forstart of rusting of Type of coating: steel base (4) Same as (2) withapproximately 45% Al l3 (5) Same as (2) with approximately 54% Al 13 (6)Same as (2) with approximately 70% Rusting of sheared edges of specimensdisregarded.

The products of our invention in the entire range from 25% to 70%aluminum in the coating, meet A.S.T.M. Specification No. A9359T for bendtest requirements. Further, in the demanding fiat lockseam test, theproducts of our invention showed no fiaking at any aluminumconcentration in the coating of from 25 to 45%. The 70% aluminum coatingshowed slight localized flaking.

Furthermore, the products of our invention having coatings in the rangebetween 25 and 45% aluminum showed only slight cracking when bent 180fiat on themselves.

The ductility and adherence of the coatings of the products of ourinvention, over the entire range of aluminum content (25 to 70%aluminum) are such that the said products can withstand commercialforming such as corrugating, bending, etc. without significant crackingor flaking.

Our improved coating may also be applied by What may be referred to as aroll-bonding procedure. As an example of this method, the surface of alow carbon steel sheet was roughened by acid etching. A thin layer oftridecyl alcohol was applied to the surface of the sheet. The sheet wasthen coated with a mixture comprising zinc powder (75%) and aluminumpowder (25 the alcohol serving to promote initial adherence of thepowder to the sheet. The sheet, with the powder coating thereon, waspassed through pressure rolls to compact the powder on the sheet, andwas then heated at 750 F. for a period of about five minutes to bond thecoating to the sheet. This method is described more fully in anapplication by Edward H. Mayer and Hilton N. Rahn, filed concurrentlyherewith.

As far as is presently known, silicon is required in producing theproduct of our invention by the hot dip process. We have found that thepresence of silicon in the bath promotes the formation of an adherentcoating, suppresses the formation of an iron-bearing alloy layer at theinterface between the strip and the coating, and assures that the alloylayer which is formed is thin and uniform. We have further found thatthe silicon content of the bath should be not less than 0.5 andpreferably about 3%, of the aluminum content. More silicon may be usedif desired. Silicon will also be present in the coating. However, thesilicon content of the coating does not, so far as we can determine,have any effect on the corrosion resistance of the coated product.Silicon is not essential to the roll bonding process.

To recapitulate, we have found that our zinc-aluminum coated ferrousproducts, wherein the aluminum content represents from 25% to 70% of thetotal coating, exhibit corrosion resistance properties superior to thoseof continuously galvanized products. Within this broad range, coatingshaving an aluminum content between 25% and 45 of the total coatings arecharacterized by their ductility and freedom from flaking, whensubjected to severe forming operations; while coatings having analuminum content between 40% and 60% of the total coating arecharacterized by their remarkable corrosion resistance, which is farsuperior to that of galvanized coatings, although at some slight loss offormability at the high end of the aluminum ranges.

Examples of articles, other than steel strip, for which theabove-described zinc-aluminum coatings would find ready application, aresteel wire, hardware and structural shapes.

While the coatings of the products of our invention consist essentiallyof the metals zinc and aluminum, other substances which do notmaterially detract from the novel and basic properties of our inventionmay be present either as impurities or as deliberate additions. Forexample, the coating may contain up to 0.3% chromium for control ofintergranular corrosion.

By the term consisting essentially of we do not wish to exclude thepresence of such substances.

All percentages shown herein, which relate to bath or coatingcomponents, represent weight percent.

We claim:

1. A ferrous base having a ductile, adherent, corrosion-resistantcoating metallurgically bonded thereto, said coating consistingessentially of 25 to 70% aluminum, balance zinc.

2. A ferrous base having a ductile, adherent, corrosion resistantcoating metallurgically bonded thereto, said coating consistingessentially of 25 to 45 aluminum, balance zinc.

3. A ferrous base having a ductile, adherent, corrosion resistantcoating metallurgically bonded thereto, said coating consistingessentially of 40% to 60% aluminum, balance zinc.

4. A ferrous base having a ductile, adherent, corrosion resistantcoating metallurgically bonded thereto, said coating consistingessentially of 25 to 70% aluminum, silicon in an amount not less than0.5% of the aluminum content, balance zinc.

5. A ferrous base having a ductile, adherent, corrosion resistantcoating metallurgically bonded thereto, said coating consistingessentially of 25 to 45 aluminum, silicon in an amount not less than 0.5of the aluminum content, balance zinc.

6. A ferrous base having a ductile, adherent, corrosion resistantcoating metallurgically bonded thereto, said coating consistingessentially of 40% to 60% aluminum, silicon in an amount not less than0.5 of the aluminum content, balance zinc.

7. A ferrous base having a ductile, adherent, corrosion resistantcoating metallurgically bonded thereto, said coating consistingessentially of 25% to 70% aluminum, balance zinc, the iron-bearing alloylayer at the interface between the ferrous base and the coating beingthin and uniform.

8. A ferrous base having an adherent, corrosion resistant coatingmetallurgically bonded thereto, said coating consisting essentially of25 to 45 aluminum, balance zinc, the iron-bearing alloy layer at theinterface between the ferrous base and the coating being thin anduniform.

9. A ferrous base having an adherent, corosion resistant coatingmetallurgically bonded thereto, said coating consisting essentially of40% to 60% aluminum, balance zinc, the iron-bearing alloy layer at theinterface between the ferrous base and the coating being thin anduniform.

References Cited UNITED STATES PATENTS 2,126,244 8/1938 Cook 29-196.52,196,034 4/1940 Schulzo -178.6 2,870,008 1/1959 Neu 75146 X 3,190,7686/1965 Wright 29196.5 X

HYLAND BIZOT, Primary Examiner.

UNITED STATES PATENT OFFICE Certificate of Correction Patent No.3,343,930 September 26, 1967 Angelo R. Borzillo et 211.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

In the drawings, Sheet 1, strike out FIGS. 1 and 2 and insert insteadthe following:

in the heading to the printed specification, lines 4 to 6, for Angelo R.Borzillo, Allentown, and James B. Horton, Bethlehem, PEL, assignors toBethlehem Steel Company, a corporation of Pennsylvania readAngelo R.Borzillo, Bethlehem, and James B. Horton, Allentown, Pa., assignors, bymesne assignments, to Bethlehem Steel Corporation, a corporation ofDelaware.

Signed and sealed this 11th day of March 1969.

[SEAL] Attest:

EDWARD M. FLETCHER, JR. Attesti'ng Oflicer.

EDWARD J. BRENNER,

Commissioner 0 f Patents.

1. A FERROUS BASE HAVING A DUCTILE, ADHERENT, CORROSION-RESISTANTCOATING METALLURGICALLY BONDED THERETO, SAID COATING CONSISTINGESSENTIALLY OF 25% TO 70% ALUMINUM, BALANCE ZINC.